As there are lot of comments here, thought about clarifying few points in that article

1) About clean separation of concerns.

A lot of customers expect you to cleanly separate your client side javascript/css/artifacts from your server side implementation. Even to an extent where you can just take it and repackage the same with minor modifications using a container like Phonegap, and distribute it for mobile devices later. HTML5's significance is beyond web - it can take your app beyond the browser.

2) About the REST Layer

Anyway you are investing in building a web application, so you need to ensure the plumbing portion is re-usable beyond your traditional 'website'. If you want to build a native phone application or a Chrome plug in tomorrow, you should be able to use the same service layer.